I start with the simplest phenomena... the first... is the phenomena of light. Early on, when light was being investigated by Newton, he thought that the light that came into the eye was like a rain of particles, like rain drops... [M]ore light meant more particles... and one kind of color light would one kind of rain drop and another... would be a different kind of rain drop... over the whole spectrum... and if we would some day have sufficiently delicate instruments, we would presumably discover that it was like a pattering... [I]t would go click, click, click when the particles came raining down. ...He also discovered ...the light from the soap bubbles or light from thin films... The brightness of reflection... depends on how thick the film is. As the film gets thicker and thinner, it gets brighter and darker. That was hard for him to understand from the point of view of particles. Finally a theory of waves was invented which explained that very easily... until we measured light very precisely... and lo and behold, to our horror, it behaved like particles.

The first law is the same for both light and material bodies; they both move in a straight line, as long as they are not deflected by an outside force.
The second law is also the same as that governing the reflection of an elastic ball from an impenetrable surface. Mechanics shows that such a ball is reflected from such a surface so that its angle of reflection equals its angle of incidence, as observed for light.
But the third law still requires a plausible explanation. The passage of light from one medium to another exhibits behavior that is totally different from a ball moving through different media.

He has begun by supposing that light has a constant velocity... the same in all directions. This... could never be verified directly by experiment... The postulate... resembling the ... furnishes us with a new rule for the investigation of simultaneity.

Newton's proof of the law of refraction is based on an erroneous notion that light travels faster in glass than in air, the same error that Descartes had made. This error stems from the fact that both of them thought that light was corpuscular in nature.

Opticks was out of harmony with the ideas of 19th-century physics. ...an exposition of the "wrong" (i.e., corpuscular) theory of light,—even though it also contained many of the basic principles of the "correct" (i.e., wave) theory. Not only had Newton erred in his choice... but also he apparently had found no insuperable difficulty in simultaneously embracing features of two opposing theories. ...by adopting a combination of the two theories at once, he had violated one of the major canons of 19th-century physics... Today our point of view is influenced by the theory of photons and matter waves, or the... complementarity of Niels Bohr; and we may read with a new interest Newtons ideas on the interaction of light and matter or his explanation of the corpuscular and undulatory aspects of light.

Having discovered the true principle, I then derived all the laws that govern the motion of light, those concerning its direct propagation, its reflection and its refraction. I reserve for particular members of our Assembly the geometrical demonstration of my theory.
I know the distaste that many mathematicians have for final causes applied to physics, a distaste that I share up to some point. I admit, it is risky to introduce such elements; their use is dangerous, as shown by the errors made by Fermat and Leibniz in following them. Nevertheless, it is perhaps not the principle that is dangerous, but rather the hastiness in taking as a basic principle that which is merely a consequence of a basic principle.

Qu. 4. Do not the Rays of Light which fall upon Bodies, and are reflected or refracted, begin to bend before they arrive at the Bodies; and are they not reflected, refracted, and inflected, by one and the same Principle, acting variously in various Circumstances?

Long ago, Sir Isaac Newton gave us three laws of motion, which were the work of genius. But Sir Isaac's talents didn't extend to investing: He lost a bundle in the South Sea Bubble, explaining later, "I can calculate the movement of the stars, but not the madness of men." If he had not been traumatized by this loss, Sir Isaac might well have gone on to discover the Fourth Law of Motion: For investors as a whole, returns decrease as motion increases.

Newton's First Law, the... Law of Inertia, refers only to bodies that are subject to no external forces. It is tempting to say that Newton postulates that such bodies "continue in the same state of motion," but such... would miss the revolutionary aspect... the First Law specifies exactly what counts as "the same state of motion." For Aristotle... a piece of aether in uniform circular motion about [earth,] the center of the universe is always in "the same state of motion," and so there would be no reason to seek out external causes... In Aristotle's physics, external causes are responsible for unnatural motion, such as a rock moving upward instead of down. So for Aristotle, the falling of a stone... requires no external cause, and the continued rotation of a sphere of fixed stars requires no external cause: this what these sorts of matter do by nature.

...the principle of the limiting character of the velocity of light. This statement... is not an arbitrary assumption but a physical law based on experience. In making this statement, physics does not commit the fallacy of regarding absence of knowledge as evidence for knowledge to the contrary. It is not absence of knowledge of faster signals, but positive experience which has taught us that the velocity of light cannot be exceeded. For all physical processes the velocity of light has the property of an infinite velocity. In order to accelerate a body to the velocity of light, an infinite amount of energy would be required, and it is therefore physically impossible for any object to obtain this speed. This result was confirmed by measurements performed on electrons. The kinetic energy of a mass point grows more rapidly than the square of its velocity, and would become infinite for the speed of light.

A ray of light, passing close to a heavy body, should, on Einstein's assumption, suffer a slight chance of direction, as if it were pulled towards the body. According to Newton's principles, there seems to be no reason why the light should be bent at all. It is possible, however, that light possesses the equivalent of weight in a material body, and, if so, the gravitational force should cause a bending similar to that predicted by the theory of relativity, but of only half the amount.

If the world is to be understood, if we are to avoid such logical paradoxes when traveling at high speeds, there are some rules, commandments of Nature, that must be obeyed. Einstein codified these rules in the special theory of relativity. Light (reflected or emitted) from an object travels at the same velocity whether the object is moving or stationary: Thou shalt not add thy speed to the speed of light. Also, no material object may move faster than light: Thou shalt not travel at or beyond the speed of light. Nothing in physics prevents you from traveling as close to the speed of light as you like; 99.9 percent of the speed of light would be just fine. But no matter how hard you try, you can never gain that last decimal point. For the world to be logically consistent, there must be a cosmic speed limit. Otherwise, you could get to any speed you wanted by adding velocities on a moving platform.

Newton's laws of motion and gravitation achieved their original success when applied to the solar system. The first definite evidence that they were applicable on a larger scale came from the study of binary stars towards the eighteenth century. In recent times the limitations of Newton's theory have become apparent. Even on the scale of the solar system, it has been challenged by Einstein's.

The laws of motion... were set out by... Newton at the beginning of... the Principia. ...[T]he key principle is ...in the Second Law... paraphrased as... the force to give an object a certain acceleration is proportional to the product of the and the .